Observers often need to keep track of moving items. Previous research has shown that tracking is much more difficult if the items appear to pour from one location to another in a substance-like manner, leading to the claim that observers can track objects but not substances (vanMarle & Scholl, 2003, Psychological Science, 14(5), 498). When a substance pours, it necessarily expands and contracts. Here we show that expansion and contraction per se inhibits tracking, regardless of whether the expansion and contraction is consistent with a substance-like movement. Four additional experiments identified two reasons why expansion and contraction inhibits tracking: 1) Items that expand and contract tend to overlap with each other and these overlaps make it hard to confine attention to just the subset of the items that the observer wishes to track. 2) Expansion and contraction create motion signals in the direction of the expansion and contraction. Such signals conflict with those motion signals that indicate in what direction the item is moving, i.e. the movement of its center. This reduces tracking accuracy, likely because it makes it harder for observers to predict the item's future location.